c# – HttpClient对象方法丢失

HttpResponseMessage response = await client.PostAsJsonAsync("api/...",user);

这是在这个使用声明(添加的标题)

using System;
        using System.Net.Http;
        using System.Net.Http.Headers;
        using System.Net.Mail;
        using System.Threading.Tasks;
        //...

        using (var client = new HttpClient())
        {
            client.BaseAddress = new Uri("WebServiceAddress");
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            HttpResponseMessage response = await client.PostAsJsonAsync("api/...",user);
            if (response.IsSuccessStatusCode)
            {
                const string result = "Thank you for your submission.";
                return result;
            }
            //...
        }

我得到的错误说

Error  4   'System.Net.Http.HttpClient'
 does not contain a definition for 'PostAsJsonAsync' and no extension
 method 'PostAsJsonAsync' accepting a first argument of type 'System.Net.Http.HttpClient'
 could be found (are you missing a using directive or an assembly reference?)

即使它在前一个项目中工作,并从该项目中直接复制.我忘了添加东西吗？

我感谢任何有关此事的帮助.